1. Field of the Invention
This invention relates to spectrometry, and more particularly to a spectral imager and associated fabrication method with graded effective spectral absorption properties in the elongate direction of a light propagation medium.
2. Description of the Related Art
Spectral imagers detect radiation emitted from or reflected by a scene area in two or more colors. Spectral imagers in general can be characterized as “multispectral” or “hyperspectral”. Multispectral devices (like the human eye) are capable of detecting the image of a scene in only a few wavebands of light, while hyperspectral devices can detect the image in many more wavebands.
Multispectral imagers can be formed in several ways, for example by having fixed filters covering several spectral bands in a scanning system, by serially placing several different filters over a broad band imaging array, or by stacking semiconductor layers containing area arrays of single detector picture elements, typically photodiodes or photoconductors, of different material compositions which absorb progressively longer wavelengths, registering the detector arrays to each other within the stack to form an array of multispectral picture elements (pixels), and making contacts to each layer's detector within each pixel. A major surface of the device is illuminated, and the amount of light absorbed by each layer in each pixel is detected from its in-pixel contact to sense the spectral content of the light, typically within two or three wavebands.
Several different approaches have been taken to hyperspectral imaging. Current imagers use a two-dimensional array of detectors. The hyperspectral image of the scene (often called a hypercube), however, is three dimensional, with two spatial dimensions and one spectral dimension. With only two dimensions available in the detector array, the third must be obtained over time with multiple frames of data from the array. This need for time multiplexing has led to two major alternative approaches to hyperspectral imaging, and one combined approach. One major type of hyperspectral imager includes systems in which the two scene spatial dimensions are collected simultaneously while the wavelength information is multiplexed over time, such as by using a rotating spectral filter wheel, a tunable Fabry-Perot interferometer, or a Michaelson interferometer (which interferes many different wavelengths to obtain an interferrogram whose Fourier transform represents the spectrum of the incident light). An alternate approach images simultaneously a single row of scene spatial information in one array spatial dimension and disperses, for instance with a prism or diffractometer, the spectral information from each of the scene elements of this row in the other array spatial dimension. The other scene spatial dimension is then scanned in a transverse direction over time.
In each of these prior approaches, hyperspectral imagery requires complex optics. A hybrid approach again employs an array of detectors preceded by a linear variable filter placed in close proximity to the array to select a different wavelength from each row of the scene to be detected by each row of pixels. To obtain the complete hypercube the scene must be scanned across the area array. This process avoids the complexity of the moving optical parts of the first approach (in which spectral information is obtained over time), but suffers from the need for delicate alignment of the filter, the need to have relatively slow optics, and filter inefficiency and difficulty of fabrication. Moreover, all of these approaches may require the detector array to be cooled to a temperature sufficiently low that the detector dark current is well below the photocurrent. This is hard to do for a wide spectral range since long wavelength detectors have, for basic physics reasons, exponentially higher dark currents than do short wavelength detectors.